1. Field of the Invention
The present invention relates to a process for treatment of aluminum-containing sodium hydroxide waste liquor produced by treating aluminum articles with a sodium hydroxide solution or the like, so as to recover recyclable sodium hydroxide. Accordingly, the present invention may be appropriately applied to the treatment of aluminum-containing sodium hydroxide waste liquor derived from, for example, the alkali washing solutions used in factories manufacturing an aluminum sash or alumite, or used for dies in aluminum extrusion factories.
2. Description of the Related Art
As the conventional treatment of aluminum-containing sodium hydroxide waste liquors discharged when treating a surface of or dissolving metal aluminum or aluminum alloys with a sodium hydroxide solution, there were known hitherto (1) the neutralization process by adding acid, and (2) the process for recovering the aluminum hydroxide and sodium hydroxide solution using the Bayer process. The above process (1) is simple in procedure, but the sludge produced by neutralization contains aluminum hydroxide as a main ingredient and includes a large amount of water, so its treatment is difficult. Further, it was not possible to regenerate the sodium hydroxide for recycle. The above process (2) required a long reaction time (for about 24 to 48 hours). Further, aluminum hydroxide was poorly separated from sodium hydroxide, and thus, a relatively large amount of aluminum was present in the recovered sodium hydroxide solution. Therefore, various problems occurred at the time of reuse of the recovered sodium hydroxide solution, such as precipitation of aluminum hydroxide and deposition on container walls as scale. If a masking agent such as sodium gluconate was used to remedy such defect, separation and recovery of the sodium hydroxide solution and aluminum hydroxide were not possible.
Under the circumstances, the present inventors and the co-workers proposed a process for recovering calcium aluminate and sodium hydroxide waste liquor, by adding calcium hydroxide [Japanese Examined Patent Publication (Kokoku) No. 54-4720]. In the process, aluminum was precipitated and separated in the form of 3CaO--Al.sub.2 O.sub.3 --6H.sub.2 O from the aluminum-containing sodium hydroxide waste liquor to regenerate sodium hydroxide. However, when the above regeneration steps were repeated, carbon dioxide gas present in the air was absorbed and accumulated in the sodium hydroxide solution to form sodium carbonate, so there was the problem that the etching ability of the regenerated solution was lowered.
Further, Japanese Unexamined Patent Publication (Kokai) No. 55-144415 disclosed a process for recovering a sodium hydroxide solution from the above sodium hydroxide waste liquor by adding aluminum sludge containing aluminum hydroxide as a main ingredient and then silicate to said waste liquor. However, the reaction between the aluminum in the waste liquor and the silicates in the above process produce zeolite, which is different from hydrotalcite produced in the process according to the present invention as mentioned below.
Japanese Unexamined Patent Publication (Kokai) No. 63-8215 disclosed a process for adding magnesium oxide to an aqueous solution containing a high concentration of sodium hydroxide, and allowing the whole to stand. The process, however, was not designed to treat aluminum waste liquor, but to remove chromium or nickel included as impurities in an industrial aqueous solution containing a high concentration of sodium hydroxide.
Japanese Unexamined Patent Publication (Kokai) No. 60-90816 disclosed a process for regenerating an aluminum-containing sodium hydroxide solution by adding a calcium compound for the treatment, whereas a magnesium compound is added in the process of the present invention as mentioned below.
Japanese Unexamined Patent Publication (Kokai) No. 4-265220 discloses a process for recovering sodium hydroxide containing aluminate ions including aluminum atoms. In the above process, however, chelating agents were used, whereas a magnesium compound is used in the process of the present invention as explained below.
Japanese Unexamined Patent Publication (Kokai) No. 2-196016 discloses a process for treating a sodium hydroxide solution in which aluminum is dissolved. In the above process, however, solid particles, such as synthetic zeolite or silicon carbide is added to remove the scale and concentrating the sodium hydroxide, whereas a magnesium compound is added in the process of the present invention as explained below.